1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method for processing multivalued image data for an area (for example, a pixel area) on a recording medium to record an image in the area by a plurality of relative movements between the recording medium and a recording head.
2. Description of the Related Art
As a technique for reducing color density unevenness and streaks in images recorded by inkjet recording apparatuses, a multipass recording method is known of completing an image to be recorded in an area on a recording medium by causing a recording head to perform recording scanning a plurality of times in the area. However, even if such a multipass recording method is employed, the last recording scanning and the next recording scanning may not be performed at the same dot recording position owing to the change in a conveyance distance of a recording medium. This causes the change in a dot coverage ratio, so that image degradation such as the change in the color density or the color density unevenness occurs.
As a technique for reducing such image degradation, Japanese Patent Laid-Open No. 2000-103088 discloses a method of dividing multivalued image data into pieces of multivalued image data for different recording scans and separately binarizing these pieces of multivalued image data. FIG. 16A is a diagram illustrating the arrangement of dots recorded on the basis of image data processed with the method disclosed in Japanese Patent Laid-Open No. 2000-103088. Referring to FIG. 16A, black circles 551 represent dots recorded in a first recording scan, white circles 552 represent dots recorded in a second recording scan, and gray circles 553 represent dots each generated by overlapping a dot recorded in the first recording scan and a dot recorded in the second recording scan.
Even if the group of dots recorded in the first recording scan and the group of dots recorded in the second recording scan are shifted in a main scanning direction or a subscanning direction, a dot coverage ratio with respect to a recording medium is not significantly changed. The reason for this is that a portion in which a dot recorded in the first recording scan and a dot recorded in the second recording scan overlap each other is newly generated and a portion in which a dot recorded in the first recording scan and a dot recorded in the second recording scan should overlap each other is eliminated.
In the method disclosed in Japanese Patent Laid-Open No. 2000-103088, however, since pieces of multivalued data corresponding to different recording scans are separately quantized, it is impossible to control the amount of overlapping of dots recorded in different recording scans. Accordingly, the number of overlapping dots may be markedly increased, and the graininess of an image may be deteriorated. In contrast, the number of overlapping dots may become too small to suppress the change in color density.
Furthermore, in the method disclosed in Japanese Patent Laid-Open No. 2000-103088, the number of times of division of multivalued data increases with the number of passes. That is, the number of pieces of multivalued data to be subjected to quantization increases. Accordingly, when this method is performed in a recording mode in which the number of passes is large, the load of quantization is markedly increased.
When the number of passes is large, the amount of conveyance of a recording medium performed between passes is relatively small. Accordingly, the amount of change in a dot coverage ratio which is caused by the change in a conveyance distance of the recording medium is small. It is therefore unnecessary to perform processing for generating a portion in which dots overlap each other so as to suppress the change in a dot coverage ratio. When the number of passes is large, it is desirable that processing for reducing graininess and the load of quantization be performed.